This module provides a number of list utility functions,
all of which take an initial code block to control their behaviour.
They are variations on similar core perl or List::Util functions of
similar names, but which use the block to control their behaviour.
For example, the core Perl function sort takes a list of values and
returns them, sorted into order by their string value.
The sort_by function sorts them according to the string value
returned by the extra function, when given each value.
